Using Generics in python

#python   #typing   #generic  

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Table of Content

Using Generics in python

Generic is a way to specify the type of data that variable can hold at use time and not when this type declare

from typing import TypeVar, Generic, List, Any

T = TypeVar("T") #(1)
def first(items: List[T]) -> T: #(2)
    return items[0]

first_item: int = first([1, 2, 4])

1. The TypeVar declare that T can any type
2. The function first get list of type T and return T

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Generic class

from typing import List, Generic, TypeVar

T = TypeVar("T")

class Items(Generic[T]):
    def __init__(self) -> None:
        self.__data: List[T] = []

    def add_item(self, item: T) -> None:
        self.__data.append(item)

    def first(self) -> T | None:
        if self.__data:
            return self.__data[0]
        return None


if __name__ == "__main__":
    items = Items[str]()
    items.add_item("a")

    other_items = Items[int]()
    other_items.add_item(1)

Demo: Generic class with multiple generic types

from typing import Generic, TypeVar

T = TypeVar("T")
U = TypeVar("U")


class MyClass(Generic[T, U]):
    def __init__(self, foo: T, bar: U) -> None:
        self.foo = foo
        self.bar = bar

    def get_foo(self) -> T:
        return self.foo

    def get_bar(self) -> U:
        return self.bar

if __name__ == "__main__":
    obj = MyClass[str, int]("name", 10)
    print(obj.get_foo())

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Reference

• Using Generics in Python
• Python’s covariance and contravariance